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It's Q19
Sorry - am using the wrong browser so can't insert hyperlinks, apparently.
http://store.aqa.org.uk/resourceZone/pdf/maths/AQA-9365-2H-W-SQP.PDF
The mark scheme starts off with: 20 x BX = 15 x 8, then continues to solve for BX and then finish to find the diameter.
Thanks.
Schifel, if you private message me with an email address, I will send you a textbook chapter on this topic with exercises and answers.
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